Besides crosstalk noise counts, there are also noise counts from re-Railey scattering noise [1]}, which is caused by the backscattering of the Railey backscattered phase reference light and detector dark counts. We estimate this re-Railey scattering noise to about 8 cps on each single photon detector, based on the detected reference counts. The dark counts for SNSPDs are 4 cps and 6 cps, with detection efficiency of 77% and 91% respectively. In total, the noise counts for each detector is about 24 cps. We then further use a 350 ps time filtering to reduce the noise probability per gate, achieving a noise count rate of \(7\times 10^{-9}\) .
